1. Field of the Invention
The present invention relates to an anisotropically pressure-sensitive electroconductive sheet. In particular, the invention relates to an anisotropically electroconductive sheet having a volume resistivity as measured in a direction within the plane of the sheet much larger than the volume resistivity as measured in the direction perpendicular to the plane of the sheet when a pressure is applied in the direction perpendicular to the plane.
2. Description of the Prior Art
In the prior art there are known various electrically conductive plastics and elastomers prepared by uniformly dispersing various kinds of electrically conductive powders or fibers, such as metal powders, carbon black, graphite powder, carbon fibers and the like in various kinds of electrically insulating plastics or elastomers as the matrix. In particular, researches have been directed to the possibilities of obtaining anisotropically electroconductive materials based on the electrically conductive plastics or elastomers, which can find specific uses, for example, as electric contact materials in certain electronic circuits.
The prior art anisotropically electroconductive materials have been prepared by the following methods, particularly in a sheet-like form.
According to one example, a plurality of sheets of an electrically conductive material and a plurality of sheets of an electrically insulating material are stacked together alternately one on top of the other, pressed together, and bonded into a block. The resultant block is sliced with parallel planes perpendicular to the plane of the stacked sheets to form striated sheets. These sheets have accordingly anisotropic electroconductivity, that is, they are conductive in the direction perpendicular to the plane of the sheet but insulating in the direction perpendicular to the direction of striation within the plane of the sheet.
According to another example, an elastomeric sheet of a rubbery material is impregnated with metal particles in a relatively small amount such that the impregnated material per se can not be electrically conductive. The resultant sheet is then employed as the anisotropically electroconductive material. When the sheet thus obtained is pressed perpendicularly on the surface, the metal particles dispersed in the matrix come into contact with each other in the direction of pressing to form pressure-sensitive electrical contact in the direction perpendicular to the plane of the sheet, while no electrical contact is formed in the directions within the plane of the sheet. One example of this product is "CHO-NECTOR", product of Chomerics Inc., U.S.A.
These anisotropically electroconductive sheets of the prior art are used widely in various kinds of electronic devices as a contact material for rapidly connecting electronic circuits with high fidelity, such as, lead wire-free integrated circuits and hybrid circuits, connection of a flexible circuit and the terminals of a printed circuit as well as an interconnector between a display unit of liquid crystals or a light emitting diode in pocketable electronic calculators, electronic watches or the like and a circuit board.
These prior art sheet materials with anisotropic electroconductivity, however, cannot be free from several problems. For example, the anisotropically electroconductive materials prepared according to the first example described above have disadvantages in high costs of manufacture and difficulties in manufacturing technology, especially when designed for use as the connection for multi-circuits or closely spaced circuits, along with the fact that the number of circuits connected therewith is limited by the number of the layers of the electrically conductive material. In addition, they are defective in that insulation breakdown in the layers of the electrically insulating material tends to take place when the sheet material is designed for use in the connection of closely spaced circuits.
The anisotropically electroconductive sheets obtained according to the second example described above have disadvantages in that insulation resistance between the circuits sometimes becomes unduely low owing to contacting pressure to form anisotropical electric contact points. Furthermore, the reliability in electric contact between circuits is insufficient when the sheets are intended for use in the connection with closely spaced circuits in a miniaturized device and accordingly are made very thin with decreased anisotropy.